The present invention relates to digital communication where Orthogonal Frequency Division Multiplexing (OFDM) is employed, and more particularly to cancellation of inter-carrier interference between OFDM sub-carriers.
In an OFDM system, a signal comprises a number of sub-carriers that are independently modulated, each by its own data. The modulation can be in accordance with a number of well-known techniques, such as Quadrature Amplitude Modulation (QAM) or n-ary Phase Shift Keying (n-PSK). The baseband signal in an OFDM system is then the sum of these modulated sub-carriers. The baseband signal is then used to modulate a main radio frequency (RF) signal. An important aspect of demodulating such a signal (thereby retrieving the underlying baseband signal) involves processing it by a Fast Fourier Transform (FFT). An advantage to communicating by means of OFDM is that it allows for communication over highly time-dispersive channels (i.e., due to multi-path propagation of a transmitted signal) using reasonable complexity at the receiver side.
The way to handle large delay spreads for a system based on OFDM is to make use of a guard interval, often referred to in the literature as a “cyclic prefix”, (“CP”). The CP is simply a copy of the last part of an OFDM symbol that is sent before the actual symbol. This is schematically illustrated in FIG. 1, which shows a number of symbols. An exemplary one of the symbols 101 includes a last portion 103 that is transmitted as a preceding cyclic prefix 105 (time flows from left to right in the figure). Other cyclic prefixes are similarly formed from end portions of their immediately succeeding symbols.
It is well-known that for a system based on OFDM the effect of the time-dispersive channel, known as inter-symbol interference (ISI), can be avoided provided that the length (i.e., duration) of the CP is at least as long as the (maximum) duration of the impulse response of the channel. Because of the ability of an OFDM system to handle large delay spreads, it is very suitable for so-called Single Frequency Networks (SFN), which might be used for broadcasting. (In a single frequency network, geographically spaced transmitters operate on a same frequency, and are time synchronized with one another.) OFDM is also becoming the choice for other types of wireless communications systems. It is used for Wireless Local Area Networks (WLAN), Broadband Access (Wi-Max), Digital Video Broadcasting (DVB), Digital Audio Broadcasting (DAB), and it has been proposed for the fourth generation (4G) of mobile communications equipment.
A major technical problem facing OFDM systems is their susceptibility to frequency offsets, phase noise, and Doppler effects when the channel is rapidly changing. These problems cause inter-carrier interference (ICI) between the OFDM sub-carriers, resulting in high bit error rates.
More particularly, ICI is caused by lack of orthogonality between the signals received on different sub-carriers. Since the orthogonality is in frequency, things like frequency error, phase noise, and Doppler spread will all cause a loss of orthogonality. The effect of frequency error and phase noise can in theory, and also often in practice, be made small enough by proper design. However, ICI caused by Doppler spread will be present even in an ideal receiver because it is caused by channel variations.
For a text-book OFDM system, the power of the ICI, PICI, caused by Doppler spread when Jakes' Spectrum is assumed is given by
            P      ICI        =                            π          2                6            ⁢                        (                                    f              D                                      Δ              ⁢                                                          ⁢              f                                )                2              ,where fD=fcv/c is the maximum Doppler frequency, and Δf is the carrier spacing between the sub-carriers. Here, fc is the carrier frequency, v is the relative speed between receiver and transmitter and c=3×108 m/s is the speed of light. When designing an OFDM system, this understanding of ICI can be used to ensure that ICI caused by Doppler effects will not be a problem by making Δf sufficiently large in relation to the expected Doppler shift.
To date most OFDM applications have been for fixed or low mobility applications (low Doppler). More recently, however, high mobility services have been targeted, (e.g., DAB, DVB-H, and 4G.) Such systems require improved performance when the experienced Doppler effects are high.
For example, Doppler shifting is an issue for DVB-H systems because DVB-H is designed to be backward compatible with the older DVB-T system, rather than being designed from scratch. The DVB-T system was originally designed for receivers with low mobility; in most cases, the receiving antenna is placed on the roof-top of a building, and therefore completely stationary. Since DVB-H is targeting highly mobile devices, one can say that the system is not properly designed. To a certain extent, the 4k mode introduced in DVB-H (in which the size of the FFT is 4k compared to the 8k FFT that is normally used in DVB-T systems) is an attempt to improve Doppler tolerance for the system.
For the DVB-H system in the United States, Doppler shifts are even more of an issue. The reason for this is twofold: First, the carrier frequency is 1.67 GHz, which is roughly twice the highest frequency under consideration elsewhere for DVB-H. Second, the bandwidth of the U.S. system is only 5 MHz. The former means that fD will be at least twice as high for the same vehicle speed, whereas the latter means that the carrier spacing Δf is reduced by a factor of ⅝ compared to the 8 MHz system.
OFDM performance in Doppler or with frequency offset is improved by using fewer sub-channels. Reducing the number (N) of sub-channels permits a corresponding decrease in the symbol duration. However, this leads to a drop in transmission efficiency because the length of the cyclic prefix cannot be reduced accordingly due to channel delay spread considerations. The cyclic prefix length is designed to cover the longest delay spread expected on the channel. As such, in normal operation, this is generally wasteful since the longest delay spread hardly ever occurs.
One method for improving OFDM Doppler performance is using more robust modulation (e.g., as in DAB). Another is adding an additional layer of coding (e.g., as in DVB-H). Yet another is introducing a special code for minimum degradation due to ICI, such as repetition coding (transmitting the same data on adjacent sub-channels). All of these schemes suffer from reduced throughput due to the smaller modulation alphabet or the increased coding overhead.
Other methods include performing ICI cancellation in the receiver. A matrix of coefficients is formed describing the ICI coupling into adjacent channels. The received signal is then multiplied by the inverse of this matrix to remove the ICI effects. The scheme is complex because many sub-carriers contribute to the ICI, especially when the signal to noise requirement is high as it is for high data rate systems using high level modulations, such as 64 QAM.
There is therefore a need for techniques for improving the performance of OFDM-based systems for use in environments in which the experienced Doppler effects are high.